This invention relates generally to turbine blades and, specifically, to a design modification in a turbine blade aft shank wall for stress reduction.
When a bucket in a power systems turbine or a blade in an aircraft engine requires a hollow core for cooling air, the proximity of the core cooling cavities to the forward and rear shank walls (coverplates) can be problematic. For example, the cooling cavity closest to the aft shank wall may create a barely sufficient, thin metal ligament between the aft wall and the cooling cavity, causing a high stress location and potentially low part life. Additionally, this situation may preclude the shank wall from undercutting the airfoil trailing edge, a desired condition for airfoil root trailing edge stress relief. In other words, the desire to have the trailing edge overhang the aft shank wall often competes with the requirement to maintain adequate wall thickness between the core cavity and the aft shank wall.
Reduction of stress, particularly in the airfoil root trailing edge, requires both thermal management of the part to reduce thermal stress, and a geometry designed to relieve the loading of the airfoil trailing edge.
In a related matter, sealing of the turbine bucket or blade is crucial, as parasitic leaks degrade engine performance. Seal pins are typically used to reduce leaks between adjacent buckets. It is therefore also imperative that the ends of adjacent shanks join relatively closely from the concave sides to the convex sides of the buckets.
This invention relieves stress at the root trailing edge of a rotating turbine airfoil while maintaining adequate wall thickness between core cavities and the aft shank wall. The invention also maintains mating side surfaces for sealing and leak reduction. More specifically, the invention utilizes a convexly curved aft shank wall that provides additional material between the aft shank wall and the nearest cooling cavity, and that also enables increased overhang of the trailing edge of the airfoil. The aft wall is curved from one side of the bucket to the other, so as to allow adjacent edges to align and thus facilitate sealing between adjacent buckets.
It will be appreciated that for some bucket applications, it may be desirable to provide a convexly curved surface on the forward shank wall as well.
In one aspect, therefore, the invention relates to a turbine bucket comprising an airfoil portion and a shank portion with a substantially horizontal platform radially therebetween, the shank portion having a forward shank wall and an aft shank wall; at least the aft shank wall convexly curved from one side of the bucket to an opposite side of the bucket.
In another aspect, the invention relates to a turbine bucket comprising an airfoil portion and a shank portion with a substantially horizontal platform radially therebetween, the shank portion having a forward shank wall and an aft shank wall; the aft shank wall convexly curved from one side of the bucket to an opposite side of the bucket; wherein the internal cooling circuit includes a plurality of cavities in the shank portion and the airfoil portion and wherein one of the cavities lies adjacent the aft shank wall; and further wherein the airfoil portion includes a leading edge and a trailing edge, the trailing edge overhanging the aft shank wall.
The invention will now be described in detail in connection with the drawings identified below.